Silica-supported catalysts for use in the polymerization of ethylene, propylene, and the copolymerization of ethylene with 1-butene, 1-hexene, 1-octene, and other alkenes are well-known. One type of these silica-supported catalysts is the thermally-activated chromium catalysts disclosed in U.S. Pat. No. 2,825,721 which are commonly called Phillips Catalysts. Another type is the silica-supported Ziegler-Natta catalysts which generally are activated by the addition of an aluminum alkyl compound. Among the silica supported Ziegler-Natta catalysts, for example, are: those formed from magnesium and titanium compounds as disclosed in U.S. Pat. No. 3,787,384; organoaluminum and vanadium compounds disclosed in U.S. Pat. No. 3,784,539; those formed from more than one transition metal compound such as zirconium and vanadium disclosed in U.S. Pat. No. 5,155,079; and those made with metallocene compounds disclosed in U.S. Pat. Nos. 4,701,432 and 5,057,475.
The polyolefins that are made with silica-supported catalysts are suitable for many applications, including injection molding and injection blow molding of thick walled bottles and other containers. However, it has been difficult to fabricate acceptable thin-layer products. For example, blown films are often unacceptable because of the presence of particles which have been identified as silica-containing catalyst residues. Thin coatings on electrical objects, such as wires and cables have been found to have electrical defects caused by residual silica particles in the coatings. These defects, known as "gel defects," are a serious problem in the industry.
While the prior art is replete with specifying the required physical properties of silicas as support for catalysts for the polymerization of olefins, there seems to be no recognition that defect-causing particles can be eliminated or greatly reduced in number and size by the use of silica with specific pore sizes. Illustrative examples of the prior art follow.
U.S. Pat. No. 3,960,826 discloses a catalyst prepared with silica xerogel support having a pore volume greater than 2.00 ml/g and a narrow pore size distribution between 300 to 600 Angstroms. The xerogel is intended for use in a particle form polymerization process to provide polyethylene resins of increased melt index values. There is no recognition by the patentees that silica with 200-400 m.sup.2 /g surface area and 2.5-3.5 ml/g pore volume are well suited for making catalysts that produce polyethylene suitable for thin layer application.
U.S. Pat. No. 4,983,694 discloses olefin polymers having melt flow ratios of 25 to 50 produced with supported catalysts, the support having average pore diameters of 20 to 300 Angstroms, wherein the catalyst particle size is kept constant. The catalyst-produced polymers had gradually decreasing molecular weight distribution with decreasing pore size of the catalyst at a constant particle size. It appears that this discovery can serve as a means to regulate the molecular weight distribution of polyolefins through the physical characteristics of the silica. However, there is no recognition by the patentees to decrease the prevalence of the defect-causing catalyst particle residues in the so-produced polymers.
U.S. Pat. No. 4,849,390 describes catalyst characteristics required to increase the bulk density of polyolefin resins, to control or eliminate the formation of dust particles, and to improve the dry flow of the particles produced by the polymerization process. According to the patent, these improvements can be obtained by using a silica with large amounts of nearly spherical particles having an average pore size of 180 to 250 Angstroms in diameter, with more than 60% of the pore size being in the range of 100 to 300 Angstroms, and a degree of resistance to disintegration by treatment of the catalyst with ultrasonic waves. As a result, this patent teaches away from the use of silica which is more susceptible to disintegration upon exposure to ultrasonic waves.
U.S. Pat. No. 4,175,170 discloses a process for the manufacture of homopolymers and copolymers of alpha-mono olefins. The process includes the use of a finely divided inorganic oxide material which has a particle diameter of from 1 to 1,000 microns, a pore volume of from 0.3 to 3 ml/g, and a surface area of from 100 to 1,000 m.sup.2 /g. These ranges include silica types which appear to be unsuitable for polyolefin processes especially those processes which produce polyethylene especially geared for thin layer applications.